[Super Newb]

In the crossed aldol reaction between acetone and 2 equiv of benzaldehyde, the LC-MS shows almost 1:1 ratio of benzalacetone and dibenzalacetone. What could be the reason of the poor conversion? Can someone advice me on how to improve on the reaction?  

The procedure is stated below:

Foremost, 25ml of NaOH solution (10g/100ml) and 25ml of ethanol were transferred into a rbf (round bottom flask).
 
The rbf was clamped to a retort stand and lowered into ice bath. The magnetic stirrer was then turned on to ensure that the solution is homogenous.
   
Next, 2.5ml of benzaldehyde and 919 ul of acetone were collected in a conical flask and mixed well before adding into the rbf. The reaction mixture was then left to stir for 20 minutes for completion, and when the reaction has proceeded to completion, TLC was taken and eluted a 2 different solvent conditions (EA/HEX = 1:4 and 1:9).
 
The crude product was then collected by suction filtration and repeatedly washed with D.I water.

[Super Newb]

I have 2 more questions

1.Why some of the protocol reported by literature suggest to add the 1/2 of the mixture of (acetone and 2 equiv of benzaldehyde) and stir for a period of time before adding the second half of the mixture? Instead of adding the all of the mixture at once.

2.  Why the reaction has to be cool under ice bath and preferably below 25 deg cel?

[opsomath]

We pretty much do this experiment in my organic 2 lab that I teach. The low temperature helps keep more complex gunky products from forming via Michael addition - if you get tarry red crap rather than pretty yellow crystals, you let it get hot. Slow addition of the reactants helps keep it chill as well.

OTOH, I am guessing that at that low temperature, you did not run the reaction to completion. Did you detect benzaldehyde left over, or does LCMS show that at all?

[orgopete]

I've too have seen this reaction performed many times. I did not have the luxury of analyzing the reaction with LC/MS. I know if stock solutions of NaOH are used, they can begin to lose their titer and the reaction slows. Since NaOH was freshly prepared, I assume this was not a factor.

I second Opsomath's question, what else did you see in the LC/MS? Acetone wouldn't be reliably analyzed as it is too volatile, but the LC/MS shows it was in excess(?). Was it in excess simply because the reaction was slower than expected and benzaldehyde was still present? Was it in excess for another reason, such as some of the benzaldehyde became oxidized and converted to benzoic acid? Did you see any benzoic acid in the LC/MS?